The aim of this paper is to analyze the dynamics of generator units during sudden impact load, direct-on-line starting loaded induction motors connected to a synchronous generator in autonomous operation. A complex mathematical model of isolated electrical grid consisting of: diesel engine, synchronous generator and motor drives have been developed. In the previous works, most often in case of direct starting of the induction motor to the grid, the torsional dynamics of the unit was not analyzed consistently, but rather the whole generator-unit was considered as a single rotating mass. Here, system with two rotating masses, one at the side of the diesel engine and the other at the side of the synchronous generator is analyzed. Ship electrical grids have such units with a diesel engine and a synchronous generator as sources. Connection of bigger induction motors to the ship's grid with such sources is the most difficult transition regime for units due to electricity loads and also due to the torsional strains in the shaft lines. This paper attempts to analyze the torsional dynamics, combining thus the knowledge in this field with the knowledge of motor drives with induction motors. The dynamics of generator-units, as well as induction motors during direct-on-line starting on isolated electrical grid is analyzed.